The subject matter relates to a device for use in the estimation of deep tissue temperature (DTT), a temperature of human or animal tissue at some distance beneath the skin. For example, the core temperature of a human body can be measured indirectly using a disposable temperature device placed on surface tissue (such as skin). The temperature of the surface tissue is read as the core temperature.
Noninvasive measurement of deep tissue temperature by means of a zero-heat-flux device was described by Fox and Solman in 1971 (Fox R H, Solman A J. A new technique for monitoring the deep body temperature in man from the intact skin surface. J. Physiol. January 1971:212(2): pp 8-10). The Fox/Solman system, illustrated in FIG. 1, estimates body core temperature by indirect means using a specially designed measurement device 10 that stops or blocks heat flow through a portion of the skin. The components of the device 10 are contained in a housing 11. The device 10 includes two thermistors 20 mounted on either side of a thermal resistance 22. The thermal resistance 22 maintains the thermistors in a spaced-apart arrangement in which the thermistors are positioned on separate sides of the thermal resistance, along a line that is generally perpendicular to a region of skin on a person's body where deep tissue temperature is to be measured. A heater 24 is disposed at the top of the device 10, over the elements 20, 22, and 24. In use, the device 10 is placed on the region of skin. With the bottom surface 26 of the device resting on the person's body, in contact with the region, the thermistors 20 measure a temperature difference, or error signal, across the thermal resistance 22. The error signal is used to drive a heater controller 30 comprising a transistor switch and a control circuit for opening and closing the switch. The controller 30 operates to minimize the error signal by causing the heater 24 to provide just enough heat to equalize the temperature on both sides of the thermal resistance 22. When the temperatures sensed by the thermistors 20 are equal, there is no heat flow through the device, and the temperature measured by the lower thermistor 20 by way of a temperature meter circuit constituted of an amplifier 36 and a temperature meter 38 is equivalent to DTT. The device 10 essentially acts as a thermal insulator that blocks heat flow through the thermal resistor 22; DTT measurement devices that operate in the same manner are termed “zero heat flux” (“ZHF”) devices. Since the heater 24 operates to guard against loss of heat along the path of measurement through the device, it is often referred to as a “guard heater”.
Togawa improved the Fox/Solman system with a DTT measurement device structure that accounted for the strong influence of dermal blood flow on heat transfer through the skin. (Togawa T. Non-Invasive Deep Body Temperature Measurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements. Vol. 1. 1979. Academic Press, London, pp. 261-277). The device, illustrated in FIG. 2, encloses Fox and Solman's ZHF design, which blocks heat flow normal to the body, in a thick aluminum housing with a cylindrical annulus construction that also reduces or eliminates radial heat flow from the center to the periphery of the device.
Fox/Solman and Togawa have shown that heat flux normal to the body is useful to control the operation of a heater that blocks heat flow through a thermal resistance. This results in a construction that stacks components, which gives the DTT measurement device a substantial vertical profile. The thermal mass added by Togawa's cover improves the stability of the Fox/Solman design. Basic engineering for heat flux measurement would suggest that a large thermal resistance in the device makes the measurement more accurate, but it will also slow the transient response rate. Since the goal is zero heat flux across the device, the more thermal resistance the better. However, additional thermal resistance adds mass and size, and also increases the time required to reach a stable temperature.
Measurement of body core temperature is desirable for many reasons. For example, maintenance of core temperature in a normothermic range during a perioperative cycle has been shown to reduce the incidence of surgical site infection; and so it is beneficial to monitor a patient's body core temperature before, during, and after surgery. Of course noninvasive measurement is very desirable, for the comfort and the safety of a patient. Deep tissue temperature measurement using a measurement device supported on the skin provides an accurate and noninvasive means for monitoring body core temperature. However, the size and mass and cost of the Fox/Solman and Togawa devices do not promote disposability. Consequently, they must be sanitized after each use, and stored for reuse. As a result, use of these devices to measure deep tissue temperature may raise the costs associated with DTT measurement and may increase the risk of cross contamination between patients. It is therefore useful to reduce the size and mass of a DTT measurement device, without sacrificing its performance, in order to promote disposability.